This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. ABSTRACT: Treponema pallidum is the noncultivatable spirochete that causes venereal syphilis, a sexually transmitted disorder and an obligate pathogen of humans. Syphilis remains a major public health problem in the United States and globally and is also well recognized as an important co-factor in the sexual transmission of the human immunodeficiency virus. Over the years, we have published a number of papers demonstrating that T. pallidum has a unique molecular architecture. The spirochete's outer membrane contains a paucity of membrane-spanning proteins while the organism's major immunogens, many of which are lipoproteins, are associated with the cytoplasmic membrane. This unusual molecular architecture protects T. pallidum from the host immune system (hence our designation of it as "the stealth pathogen"), but it also poses physiological issues that are far from understood, especially since the bacterium's genomic sequence does not contain orthologs for well-characterized OM proteins (e.g., porins) from gram-negative bacteria. Part of the solution to the riddle of nutrient transport across the T. pallidum outer membrane has been provided by our discovery of TP0453, an outer membrane-anchored lipoprotein with amphipathic helices. Given the limited means for nutrients to cross the T. pallidum outer membrane, we hypothesize that it is in very close proximity to the cytoplasmic membrane which contains the transporters needed to shuttle molecules into the cytoplasm. We believe that cryo-electron tomography will give us a much more accurate picture of the physical relationship of the outer and cytoplasmic membranes, thereby furthering our efforts to understand the physiologic aspects of T. pallidum-host interactions at the molecular level. In the previous reporting period, initial cryo-tomograms showed placement of the peptidoglycan layer that was unexpected, and cell-end specializations that are conserved between cells, and different form those in T. dentocola.